The use of biomass fuels is considered more and more advantageous, as biomass fuels are CO2 neutral, i.e. they discharge the same amount CO2 when burned, as they absorbed from the air while growing. Unfortunately, alkali metals and earth alkali metals are present in relatively large amounts in flue gases from burning of biomass or biomass fuel such as straw, woodchips and wood pellets.
A common method to catalytically reduce NOx in flue gasses is the selective catalytic reduction (SCR) using ammonia (NH3) as reductant. The production of NOx occurs in practically any high temperature process regardless of the fuel since NOx is formed by oxidation of atmospheric N2 in a flame or in a cylinder of a car engine. N2 is harmless, and constitutes around 75% of the atmosphere. The nitrogen in the fuel is of lower concern for NOx emission. Generally, NOx is an environmental problem, including acid rain formation; NOx is also considered to be harmful for human and animal health.
Established catalysts for SCR comprises e.g. oxides of V2O5 and MoO3 or WO3 supported on TiO2, which possess a very high catalytic activity.
However, these catalysts are not suitable for SCR of NOx of flue gases containing large amount of alkali and earth alkali metals, such as flue gases from biomass fuels. When these conventional catalysts are loaded with alkali earth metals or alkali metals, especially with potassium, this results in (i) a considerable decrease of catalytic activity, as well as (ii) to a shift of the maximum catalytic activity towards lower temperatures, both features that are highly undesired. In particular, the relative activity of a vanadium based catalyst decreases severely when the catalyst is poisoned with alkali metals.
It is assumed that both the alkali earth and alkali metals deactivate the conventional SCR catalyst by destruction of the essential acid sites on the surface of the catalyst (J. P. Chen, R. T. Yang, J. Catal. 125 (1990)411; Y. Zheng, A. D. Jensen, J. E Johnsson, Appl. Catal. B 60(2005)253). The severity of deactiation is proportional to the basicity of the metal oxides, where potassium oxide—due to its significant presence in the biomass fly ash combined with its high basicity—commonly constitutes the main problem.
It has been reported by A. L. Kustov, M. Yu. Kustova, R. Fehrmann, P. Simonsen, Appl. Catal. B 58(2005)97 that a catalyst having vanadium pentoxide (V2O5) supported on sulphated zirconium dioxide (ZrO2) reveals a higher resistance towards alkali poisoning than V2O6 supported on titanium dioxide (TiO2). However, a drawback in the use of vanadium-based catalysts in the SCR of flue gases from biomass is that vanadium is both more expensive and, except for Cr, more toxic than many other catalysts.
Consequently, there is a need for a relatively cheap, robust and non-toxic catalyst suitable for the selective catalytic reduction (SCR) of NOx in flue gases derived from burning biomass.